Fabry-Perot interferometers are used as optical filters and in spectroscopic sensors, for example. A Fabry-Perot interferometer is based on parallel mirrors, whereby a Fabry-Perot cavity is formed into a gap between the mirrors. The pass band wavelength of a Fabry-Perot interferometer can be controlled by adjusting the distance between the mirrors i.e. the width of the gap. It is common to use micromechanical technology for producing Fabry-Perot interferometers. Such a solution is described e.g. in patent document FI95838.
Prior art structure of a micromechanical interferometer usually includes layers of silicon and silicon oxide or silicon nitride, wherein mirrors of the interferometer have silicon oxide layer(s) or silicon nitride layers between silicon layers. A movable mirror is provided by removing a sacrificial layer, which has initially been formed between two mirror layers. The sacrificial layer may be e.g. silicon dioxide, which can be removed by etching with hydrofluoric acid (HF), for example. In order to allow the etching substance to reach the sacrificial layer, holes are provided in the movable mirror. The position of a moveable a mirror is controlled by applying voltage to electrodes, which are included in the mirror structures.
The micromechanical production technology allows series production of interferometers. However, while the micromechanical interferometers of the prior art have good performance in short wavelength ranges of visible light and near-infrared radiation, whereas their performance is worse at the longer wavelengths i.e. infrared range, especially in the range over 5 μm. This is due to the fact that silicon oxide and silicon nitride have relatively high attenuation at these longer wavelengths.
A further disadvantage of the prior art interferometers relates to a requirement to use antireflection coating in order to compensate the refractive index of the substrate. The desired compensation can usually be achieved only within a restricted range of wavelengths, whereby the interferometers may not have the required performance on a wider range of wavelengths.
There may also exist problems if an aperture on the interferometer is provided by using a material layer which is deposited at the mirror side of the substrate. In such a case the mirror structure is not symmetric due to a step which is formed by the aperture layer, and as a result the mirror layers may not be straight.